Alex Martelli <[EMAIL PROTECTED]> wrote: > Ben Finney <[EMAIL PROTECTED]> wrote: > > How can a (user-defined) class ensure that its instances are > > immutable, like an int or a tuple, without inheriting from those > > types? > > You can make a good start by defining __setattr__, __delattr__ (and > __setitem__ and __delitem__ if your class is a container) to raise > exceptions. > [...] > Do not define any of the in-place operator special methods, such as > __iadd__ and friends (or, if you're inheriting from a class that > does define them, override them to raise exceptions).
That sounds more Pythonic than hacking __slots__. Thanks. > Remember that your redefined __setattr__ IS "in place" even when > you're initializing your istance, so remember to delegate attribute > setting to the superclass (the other special methods mentioned above > are less likely to byte you). So, for a class that needs to set attributes in __init__ (but after that, become immutable), how do I get around this? Should I make a _FooFunctionality class, and then inherit from that to make Foo as the immutable class that actually gets exported? > You will probably want to define __hash__ and __eq__ if you're going > to the trouble of making instances immutable. Good tip, thanks. -- \ "I'm having amnesia and d?j? vu at the same time. I feel like | `\ I've forgotten this before sometime." -- Steven Wright | _o__) | Ben Finney -- http://mail.python.org/mailman/listinfo/python-list
